The invention relates to a linear guide for a coordinate measuring machine, and to a coordinate measuring machine.
The prior art has disclosed various linear guides for a coordinate measuring machine for measuring workpieces. Such coordinate measuring machines may be designed as portal-type measuring machines. Portal-type measuring machines comprise a portal which has two vertical columns and a cross beam which connects the two columns in an upper region, which portal is mounted, so as to be movable in a horizontal direction, on a main body for supporting the workpiece to be measured. A measuring slide is mounted so as to be movable along the cross beam, in which measuring slide there is mounted a quill, which is movable in a vertical direction. On a lower end of the quill there is arranged a sensor, in particular a tactile sensor, by means of which a surface of the workpiece can be probed. By means of the described portal mechanism, the sensor can be moved in all coordinate directions x, z, y relative to the workpiece to be measured.
Guidance of the portal, for example along a y axis, may be realized by means of one or more linear guides. A linear guide of said type is presented for example in U.S. Pat. No. 9,109,747 B2. A linear movement device is described which has a base with a surface for supporting one or more workpieces and an instrument. The instrument is arranged so as to be movable relative to the base by means of a multiplicity of guide rails, such that a movement of the instrument relative to the base along three axes is made possible. At least one of the guide rails comprises an elongate guide rail element which is composed of a multiplicity of elongate, basically planar, web elements. Each of the web elements has a length and has a pair of oppositely situated side edges which run along the length. The web elements are connected to one another at at least one or more of the side edges in order to form the elongate guide rail element. The guide rail element comprises a ceramic material.
It is also known from the prior art to use fluid pressure bearings, in particular air bearings, as a guide, which fluid pressure bearings run on the main body, for example on a granite plate that has been machined with high precision. Such solutions permit exact measurement results in the measurement of the workpiece. Such air bearings can however accommodate a force only in one direction, such that, for example for the guidance along the y axis, a multiplicity of air bearings and high-precision surfaces may be required. Such solutions can thus lead to high outlay in terms of apparatus and high costs. Furthermore, air bearings can exhibit very soft characteristics in the presence of dynamic forces such as can arise for example during an acceleration of the portal. In particular in the presence of contaminated ambient air, for example owing to contaminants in a workshop area such as oil mist and metal dust, this can lead to a high level of wear and to failures.
Also known from the prior art are portal-type measuring machines which have a main body composed of cast iron, to which there are attached linear guides in the form of recirculating ball guides. Such guides can exhibit a high level of stiffness even in the presence of dynamic forces. Recirculating ball guides are furthermore considerably less sensitive to contaminants than air bearing guides, for example.
The use of a main body composed of granite may however be advantageous in relation to cast iron, because granite exhibits less thermal expansion and has a high mass and is constant in terms of shape and dimensions, in particular is not subject to any ageing processes. Recirculating ball guides are however generally manufactured from steel. The coefficient of thermal expansion of steel however differs considerably from the coefficient of thermal expansion of granite, such that bending of the granite plate in the region of the guide, and thus inaccurate measurements, can occur.
It is therefore an object of the present invention to provide a linear guide and a coordinate measuring machine which at least substantially avoid the disadvantages of known systems. In particular, it is sought to make possible a high level of stiffness, low thermal expansion with simultaneous insensitivity, in particular suitability for workshop environments.